js-on-sx: arrays inherit from Array.prototype on lookup miss

js-get-prop for SX lists fell through to js-undefined for any key
not in its hardcoded method list, so Array.prototype.myprop and
Object.prototype.hasOwnProperty were invisible to arrays.
Switched the fallback to walk Array.prototype via js-dict-get-walk,
which already chains to Object.prototype.
built-ins/Array: 14/45 → 16/45. conformance.sh: 148/148.

lib/js/runtime.sx

@@ -2682,7 +2682,7 @@
             ((= key "copyWithin") (js-array-method obj "copyWithin"))
             ((= key "toReversed") (js-array-method obj "toReversed"))
             ((= key "toSorted") (js-array-method obj "toSorted"))
-            (else js-undefined)))
+            (else (js-dict-get-walk (get Array "prototype") (js-to-string key)))))
         ((= (type-of obj) "string")
           (cond
             ((= key "length") (len obj))